Technical Abstract:
Controlling johnsongrass (Sorghum halepense) in sugarcane is vital to achieve economically acceptable yields and to maintain ratoons. Seedling johnsongrass can be controlled through application of preemergence herbicides and inter-row cultivation. However, once rhizome johnsongrass becomes established within the sugarcane drill, the number of control options become limited. Research was conducted at the USDA-ARS Sugarcane Research Unit in Houma, LA, to evaluate the effects of crop management practices on johnsongrass control and sugarcane production over a four-year sugarcane production cycle. The locations tested had a history of johnsongrass infestation. Tillage frequency was compared at three levels: conventional tillage (four inter-row tillage treatments each spring), reduced tillage (two inter-row tillage treatments), and no-till. Three herbicide practices were compared: broadcast application of pendimethalin plus metribuzin (2.8 kg ha-1 plus 1.1 kg ha-1), banded application to sugarcane drill (90 cm band on a 180 cm row), and no herbicide application. Three post-harvest residue management practices were compared: complete removal through burning, partial removal through brushing residue from the row top into the wheel furrow, and no removal. Treatments were arranged in a split-plot design with tillage treatments as the whole plot and herbicide and residue treatments as sub-plots. In the first-ratoon crop (second year), johnsongrass density was greatest when no herbicide was applied (13 tillers m-1 row) and least with conventional tillage and broadcast herbicide application (1.7 tillers m-1 row). When post-harvest residue was not removed, johnsongrass density was less (4.2 tillers m-1 row) than complete removal by burning (6.0 tillers m-1 row), with partial removal being intermediate (5.1 tillers m-1 row). Johnsongrass density increased with all treatments, reaching a maximum of 22 tillers m-1 row in the third ratoon (fourth year) when no herbicide was applied and no-till was practiced compared to 9.7 tiller m-1 row under conventional tillage and broadcast herbicide application. Not removing harvest residue suppressed johnsongrass (14 tillers m-1 row) compared with removal by burning (20 tillers m-1 row) and partial removal (16 tillers m-1 row). In plant cane (first year), no-till reduced cane yields by 3 to 4% compared to reduced and conventional tillage. This increased to 10 to 28% yield losses in first ratoon, 24 to 45 % losses in second ratoon, and 52 to 76% losses in third ratoon, depending on herbicide usage. When conventional tillage was used, banding herbicides resulted in similar yields to broadcast application in all four years. When either no-till or reduced tillage was used, sugarcane yields were less when herbicides were band-applied compared with broadcast in the second- and third-ratoon crops. Yields were always least in all three ratoon crops when no herbicide was used. Results of this study showed that in sugarcane fields with history of johnsongrass infestation, the number of tillage applications can be reduced without sacrificing yield so long as herbicides are broadcast applied. However, a no-till approach is not advised as yields were always reduced regardless of herbicide application.